Eukaryotic cell lines can be genetically modified to express one or more desired proteins. Current selection and screening procedures to identify a clonal cell line with the requisite expression characteristics for regulated expression or production are tedious and time-consuming. For example, in Chinese hamster ovary (CHO) cells, the classical approach to achieve maximal expression involves the use of mutant cell lines and a gradual increase in the selection pressure over several months for a co-transfected selection marker such as dihydrofolate reductase. (Kaufman and Sharp, 1982; Schimke et al., 1982) While new approaches to the problem include the identification of rare sites on a chromosome with high transcriptional activity, combined with targeted integration and the improvement of selection and of screening procedures (Fussenegger et al., 1999), these are nevertheless all labor-intensive processes.